1

Supplementary Information

Solar-driven simultaneous steam production and electricity

generation from salinity

Peihua Yang‡, Kang Liu‡, Qian Chen, Jia Li, Jiangjiang Duan, Guobin

Xue, Zisheng Xu, Wenke Xie, Jun Zhou*

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and

Technology, Wuhan 430074, China.

E-mail: jun.zhou@mail.hust.edu.cn (Tel: +86 27 87792553)

‡These authors contributed equally to this work.

Table of content:

1. Experimental details

2. Supplementary Notes

3. Supplementary Figures

4. Supplementary References

Electronic Supplementary Material (ESI) for Energy & Environmental Science.This journal is © The Royal Society of Chemistry 2017

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1. Experimental details

CNT ink preparation.1 Firstly, 3 g commercial CNTs were dispersed in 120 mL

mixed H2SO4 and HNO3 (H2SO4/HNO3 = 3/1, v/v) solution by sonication for 10

minutes. Then the CNT-acid mixture was refluxed while stirring vigorously for 2

hours in 90 oC oil bath, and cooled at room temperature after the refluxing process.

The CNTs extracted from the residual acids were purified by repeating the processes:

diluting with deionized water, centrifuging and decanting the solutions. When the

supernatant was neutral, the treated CNT was re-dispersed in deionized water with the

concentration of 10 mg mL-1 to form CNT ink.

CNT filter paper preparation. A piece of filter paper (WhatmanTM 1004-150 Grade

4 Qualitative Filter Paper, Pore Size: 20-25 µm, thickness: 0.2 mm) with designed

profile (Fig. S1) was immersed in the CNT ink for 5 minutes. After fully adsorbed

CNT, the filter paper was taken out and heated at 120 oC for 30 minutes to reinforce

the association of the filter paper and CNT. The CNT modified filter paper was stored

in deionized water for further use.

The hybrid solar system fabrication. The structure and components is shown in Fig.

S2. Two pieces of organic glass (thickness 1 mm) were cutting into required mould.

Combing with homemade Ag/AgCl electrodes (thickness 0.3 mm), gaskets (thickness

0.5 mm), Nafion membrane (N117, thickness 0.18 mm), and the CNT filter paper

(with a tail of 3 mm × 1.5 cm), the device was constructed tightly by screws. The joint

of each component was sealed by polydimethylsiloxane (PDMS, A/B = 10/1, v/v).

Eventually, the effective area of Nafion membrane and Ag/AgCl electrodes was

controlled at 1 cm2 (1 cm × 1 cm). The Ag/AgCl electrode (Fig. S4) was made by

electrochemical oxidation of commercial silver foam in 0.6 mol L-1 NaCl at 1.6 V for

2 minutes using Pt as counter electrode.

Characterizations. The morphology and structure of the CNT filter paper and

electrodes were characterized by scanning electron microscopy (SEM, FEI Nova

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Nano450). The light adsorption of CNT filter paper were performed via Shimadzu

UV-Vis NIR Spectrophotometer (UV-3600). The steam generation was measured by

floating the hybrid device in a beaker with 0.6 mol L-1 NaCl solution as simulated

seawater. The solar light was supplied by a solar simulator (Newport). Different light

intensity was obtained by collecting lens. The mass change of water was measured

and recorded by a high accuracy balance (Mettler-Toledo, ME204E). The IR images

of the hybrid device under solar illumination were taken by with an infrared camera

(Flir E40). The concentration of NaCl in the CNT filter after solar irradiation was

analyzed by ion chromatograph (881 Compact IC pro, METROHM). The electrical

measurements were conducted with an Autolab PGSTAT302N electrochemical

workstation. The current-voltage curves of the hybrid device were performed at a scan

rate of 100 mV s-1.

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2. Supplementary Notes

Note 1. Solar thermal efficiency

The solar thermal efficiency is calculated as:2

(1)𝜂𝑡ℎ =

�̇�ℎLV

𝐶opt𝑞𝑖

where is the mass flux, Copt is the optical concentration, qi is the nominal direct �̇�

solar irradiation 1 kW m-2, and hLV is total enthalpy of liquid-vapor phase change

(including sensible heat and phase-change enthalpy), can be calculated as:

(2)ℎ𝐿𝑉 = 𝜆 + 𝐶∆𝑇

where λ is latent heat of phase change (2260 kJ kg-1), C is specific heat capacity of

water (4.2 kJ kg-1 K-1), and ΔT denotes the temperature increase of the water.

Note 2. Salinity power

As shown in Supplementary Fig. 5, under a concentration gradient, a potential drop

is generated by the redox reaction on the electrode (Eredox), and only the diffusion

potential (Ediff) is contributed by the cation selective membrane. According to the

previous works, in the NaCl electrolyte:3

(3)𝐸𝑟𝑒𝑑𝑜𝑥 =

𝑅𝑇𝐹

ln (𝑐𝐻𝛾𝐻

𝑐𝐿𝛾𝐿)

(4)𝐸𝑑𝑖𝑓𝑓 = (2𝑡 + ‒ 1)

𝑅𝑇𝐹

ln (𝑐𝐻𝛾𝐻

𝑐𝐿𝛾𝐿)

where t+, R, T, F, , cH, and cL represent the transference number of cations, gas

constant, temperature, Faraday constant, activity coefficient of ions, high and low ion

concentrations, respectively. t+ = 1 for complete cation selectivity, while t+ = 0 for

complete anion selectivity. The voltage detected by sourcemeter is the sum of Eredox

and Ediff, which can be expressed as:4

(5)𝐸𝑡𝑜𝑡𝑎𝑙 = 𝐸𝑟𝑒𝑑𝑜𝑥 + 𝐸𝑑𝑖𝑓𝑓 = 2𝑡 +

𝑅𝑇𝐹

ln (𝑐𝐻𝛾𝐻

𝑐𝐿𝛾𝐿)

The maximum output power of the device can be determined from the current-

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voltage curves:

(6)𝑃𝑚𝑎𝑥 =

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𝐸𝑂𝐶𝐼𝑆𝐶

where EOC and ISC are the open-circuit voltage and short-circuit current, respectively.

In the reverse electrodialysis system, the current from diffusion process can be

expressed as:5

(7)𝐼𝑑𝑖𝑓𝑓 = 𝐹𝐴𝐷

∂𝐶∂𝑥

where A is the diffusion cross area, D is ion diffusion coefficient which is a

temperature sensitive parameter, and is the concentration gradient.

∂𝐶∂𝑥

Note 3. Energy efficiency analysis

According to previous literature, the free energy change from mixing a

concentrated and a diluted solution can be written as:6, 7

(8)∆𝐺 = 2𝑅𝑇𝑉𝐻[𝑐𝐻𝑙𝑛

𝑐𝐻(1 + 𝜑)

𝑐𝐻 + 𝜑𝑐𝐿+ 𝜑𝑐𝐿𝑙𝑛

𝑐𝐿(1 + 𝜑)

𝑐𝐻 + 𝜑𝑐𝐿]

where ΔG is the free energy (J), V the volume (m3), and φ=VL/VH. H refers to the high

concentration solution and L to the low concentration solution. When the hybrid

device arrives to steady-state under solar irradiation, steam generates on the light

absorber with a rate of v (m3 (m-2 s-1)), the theoretical power density of the formed

salinity P (W m-2) can be expressed as:

(9)𝑃 =

∆𝐺𝑣𝑉𝐻

When φ is large enough, the value of ΔG will reach to a limiting value for a certain VH.

Thus, we can calculate the efficiency of salinity power from solar energy and

generated electricity form salinity gradient power.

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3. Supplementary Figures

Figure S1. Characterization of the CNT modified filter paper. (a, b) SEM images

of the CNT modified filter paper and (c) its photograph. (d) Contact angle (CA) of a

typical CNT paper, indicating the CNT modified filter paper is superhydrophilic. (e)

Adsorption spectrum of CNT filter paper. The inserted background is the solar

irradiation spectrum.

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Figure S2. Construction of the hybrid device. Two pieces of organic glass

(thickness 1 mm) were cutting into required mould. Combing with homemade

Ag/AgCl electrodes (thickness 0.3 mm), gaskets (thickness 0.5 mm), Nafion

membrane (N117, thickness 0.18 mm), and the CNT filter paper (with a tail of 3 mm

× 1.5 cm), the device was constructed tightly by screws.

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Figure S3. Experimental setup when measuring photothermal performance.

Because the main components are plastic, the device can float directly on seawater. A

layer of aluminum foil was pasted on the surface of the device with only the CNT

paper area (1 cm × 1 cm) exposing to sunlight during the solar desalination

experiments, avoiding interfere of sunlight absorption around the CNT filter paper to

the experimental results. Scale bars are 1 cm in both.

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Figure S4. SEM image of the homemade Ag/AgCl electrode. The electrode shows

a porous structure. Scale bar is 100 µm.

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Figure S5. Sodium ion selective performance of Nafion membrane. (a) Schematic

illustration of the potential difference measured across given salinity gradient. (b)

Current-voltage curves of Nafion membrane in 6 M | 0.6 M NaCl gradient using

Ag/AgCl electrode.

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Figure S6. Temperature effect to the performance of Nafion membrane. Current-

voltage curves of Nafion membrane in 6 M | 0.6 M NaCl gradient at different

temperatures. The voltage shows a litter increase with the temperature, which may be

affected by transference number of cations and ions activity coefficient under high

temperature,8, 9 on the basis of Eq. 5. High temperature facilitates ion diffusion in

Nafion membrane, thus promotes the current.

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Figure S7. The output power of the hybrid device at different solar irradiations.

At 1 and 2 kW m-2 illumination, the stable maximum output power of the hybrid

device reaches 0.5 and 1.1 W m-2, respectively.

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Figure S8. The effect of membrane thickness on the performance of the device. (a)

The open-circuit voltage of the device under one solar irradiation. (b) Current-voltage

curves of the device under one solar irradiation. The results show that, with the

decrease of the thickness, the output voltage will decrease a little because of the

reduced effective concentration difference, but the current increases because of the

reduced resistance. The output power increases from ~ 0.5 to ~ 0.55 W m-2.

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Figure S9. The electricity output of the device with different tail width under one

solar irradiation. (a) The open-circuit voltage of the device. (b) Current-voltage curves

and (c) output power of the hybrid device. Using 0.5 mm width tail, the device

maximum power increased to 1 W m-2 under one solar irradiation, which is

comparable to the device with 3 mm width tail under two solar irradiation. (d) The

evaporation induced mass loss of water for the device with 0.5 mm width tail. Inset

shows the IR thermal images of the surface temperature of the CNT filter paper under

solar irradiation.

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Figure S10. The experiment setup for measuring the water weight change rate

under natural solar irradiation. The device was covered by a layer of insulating

foam except the CNT paper area.

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Figure S11. A typical current-voltage curve of the large-scale hybrid device in 6

M | 0.6 M NaCl gradient. An open-circuit voltage of 100 mV and a short-circuit

current of 300 mA was obtained, generating a maximum output power of 7.5 mW.

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Figure S12. The temperature and relative humidity around the experiment setup

from 9:00 to 15:00 on July 30, 2017. The ambient temperature is around ~34 oC and

relative humidity is ranging from ~ 30% RH to ~ 60% RH.

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2010, 9, 1215-1224.3. D.-K. Kim, C. Duan, Y.-F. Chen and A. Majumdar, Microfluid. Nanofluid.,

2010, 9, 1215-1224.4. F. E. Kiviat, Science, 1976, 194, 719-720.5. A. J. Bard and L. R. Faulkner, Electrochemical methods: fundamentals and

applications, Wiley New York, 2001.6. C. Forgacs, Desalination, 1982, 40, 191-195.7. G. Z. Ramon, B. J. Feinberg and E. M. V. Hoek, Energy Environ. Sci., 2011, 4,

4423-4434.8. R. W. Allgood, D. J. Le Roy and A. R. Gordon, J. Chem. Phys., 1940, 8, 418-

422.9. H. L. Yeager, B. O'Dell and Z. Twardowski, J. Electrochem. Soc., 1982, 129,

85-89.